The present invention relates to a method of microbial enhanced oil recovery.
When oil is present in subterranean rock formations such as sandstone or chalk, it can generally be exploited by drilling into the oil-bearing measures and allowing existing overpressures to force the oil up the borehole. This is known as primary removal. When the overpressure approaches depletion, it is customary to create an overpressure, for example by injecting water into the formations to flush out standing oil. This is known as secondary removal.
However, even after secondary removal, a great deal of oil remains in the formations; in the case of North Sea oil, this may represent up to 75% of the original oil present Of this remaining oil probably more than half will be in the form of droplets and channels adhering to the rock formations that have been water-flooded and the remainder will be in pockets which are cut off from the outlets from the field. The present invention is concerned with the exploitation of the accessible but adhering oil remaining in the rock formations.
A number of enhanced oil recovery methods have been proposed, to address this objective. One approach is to combine pressure with a change in viscosity of the oil and/or water present. Thus, a diluent or CO2 or steam is added to the reservoir to reduce the viscosity of the oil, thereby allowing it to be freed. Alternatively, viscosity-increasing additions such as polymers may be added to the injection water so that the oil is preferentially dislodged. However, the application of CO2 is disadvantageous due to scale formation, the use of steam is only effective in shallow reservoirs of low temperature while the other additives are very costly.
Another approach is to alter the surface tension and capillary forces so that the water under pressure is more accessible to the pores and channels. This may be achieved by alkaline flooding or by means of surfactants. However, these approaches also tend to be costly.
Another approach is in situ combustion. This entails pumping air or oxygen into the formation and igniting the gas/oil present. In theory, the heat produced will mobilise the lighter fractions as a combustion front moves steadily through the formation, with the heavier tars burning. In practice, however, it is almost impossible to control the progress since the gases tend to rise while the water present sinks, resulting in an uneven combustion front.
A fourth approach is microbial enhanced oil recovery (MEOR). This entails the use of micro-organisms such as bacteria to dislodge the oil, and a number of systems have been proposed. In the case of unconsolidated measures, such as oil shales, the oil bearing rock may be pumped as an aqueous slurry to surface settling tanks or reservoirs where it is subjected to aerobic bacteria, as disclosed in U.S. Pat. No. 2,907,389. The availability of oxygen allows the bacteria to multiply, using the oil as a carbon source. In so doing, the bacteria produce surfactants which act to free the oil as droplets. The droplets of oil are less dense than water and so float to the surface. The oil is then removed. Unfortunately, the system cannot conveniently be applied to consolidated rock formations, particularly when they are undersea.
In situ MEOR methods generally fall into two categories, aerobic bacteria systems, as described typically in U.S. Pat. No. 3,332,487 and anaerobic bacteria systems as described in WO 89/10463.
The very existence of oil in a formation means that there cannot be present any anaerobic bacteria which will feed on oil under the prevailing conditions. Thus, in anaerobic bacteria systems, it has been assumed that a source of carbon or xe2x80x9cfoodxe2x80x9d must be supplied. However, under these circumstances, the bacteria selected (either deliberately or naturally) will be those most suited under the prevailing conditions to the consumption of the particular food employed. They will not be specifically adapted to have an effect on oil and therefore their action on oil will be as it were a by-product and these anaerobic systems therefore tend to be very slow in achieving the desired liberation of oil.
The absence of any oxygen in oil bearing formations means that if an aerobic system is to be used, then oxygen must be supplied. However, when aerobic bacteria are used and oxygen (or air, containing oxygen) is injected into the formation, the situation is far from satisfactory. Firstly, there is an immediate separation into a gaseous and an aqueous phase, which makes control of the system very difficult and in practice, limits the system to a batch-type operation. Secondly, a great deal of heat is generated, which, in view of the oxygen-rich gaseous phase and the readily available combustible material, presents a considerable risk of explosion. A cooling medium must therefore also be employed.
One way of addressing this is described in WO 92/13172. This reference describes a method of microbial enhanced oil recovery for recovering oil from an oil-bearing second rock formation, the formation including an inlet at a first location and an outlet at a second location, the method comprising: injecting water containing a source of oxygen, capable of yielding at least 5 mg/l free oxygen, into the formation at the first location spaced apart from the second location, allowing micro-organisms, which are either already present in the formation or which are introduced simultaneously with the oxygen containing injection water to multiply using the oil as their main carbon source and the oxygen from the injection water as their main oxygen source thereby establishing a biomass layer which acts to dissociate the oil from the rock formation, the dissociated oil then being removed via the outlet by the injection water.
In this system, on the far side of the injection well, the oxygen becomes the growth limiting factor due to the consumption of oxygen by the micro-organisms. The rate of growth of micro-organisms is of course dependent on the available oxygen. In this system maximum growth is desired and therefore it is desirable to maintain a high oxygen concentration in the injection water (and clearly also in advancing biomass layer). Field trials in Austria in recent years have shown that by the use of this system, it has been possible to obtain a threefold increase in oil production.
However it has been found that this system is effective over only a relatively short distance from the point of oxygen injection. Furthermore, it would be desirable to avoid the use of injected oxygen in order to avoid problems of corrosion. This is particularly acute in situations where the equipment susceptible of corrosion is remote and/or difficult and costly to replace.
It is known that sulphate reducing bacteria (SRB), nitrate reducing bacteria (NRB), iron-reducing bacteria (IRB) and acetogenic bacteria can grow anaerobically on oil if they have the necessary inorganic nutrients. Laboratory studies have shown that oxygen can be substituted by certain growth factors e.g. vitamins in order to achieve a reduction in residual oil concentration.
It is therefore an object of the present invention to produce oil by a method of microbial enhanced oil recovery which preferably avoids the use of injected oxygen. It is a further object of the invention to provide a method of microbial enhanced oil recovery which is effective over greater distances.
According to the invention, there is provided a method of microbial enhanced oil recovery for recovering oil from an oil-bearing rock formation, the formation including an inlet and an outlet, the method comprising: injecting water containing a source of vitamins, phosphates and an electron acceptor into the formation, allowing micro-organisms in the form of anaerobic or facultative anaerobic bacteria, which are either already present in the formation or which are introduced simultaneously with the vitamins, phosphates and electron acceptor to multiply using the oil as their main carbon source thereby establishing a biomass layer which acts to dissociate the oil from the rock formation; and removing the dissociated oil via the outlet.
Preferably, the electron donor is nitrate added as calcium, potassium, sodium or ammonium salt though sulphates could be employed. Preferably, the inlet is at a first location and the outlet is at a second location spaced apart from the first location, though the inlet and outlet could be one and the same, e.g. the production well. Preferably, substantially no oxygen is supplied with the injection water.
The bacteria are oil degrading anaerobic and/or facultative anaerobic bacteria. Preferably, the bacteria substantially comprise species of SRB, NRB, IRB and/or acetogenic bacteria. Preferably, the vitamins comprise one or more or a blend of: B12, biotin, folic acid, nicotinic acid, ammnobenzoic acid, calcium pantothenate, pyridoxine HCL, riboflavin, Thiamine and Thioctic acid. The vitamins etc. are preferably introduce continuously into the formation. Alternatively, they may be introduced periodically for example once per day.
Preferably, the individual concentration of the vitamins in the injection water is in the range of 1 to 1000 xcexcg/l and more preferably in the range of 10 to 100 xcexcg/l. Preferably, the superficial velocity of the injection water through the oil-bearing formation is between 0.1 and 15 m/day, more preferably between 0.3 and 2 n/day.
As the displaced oil is washed forwards, the micro-organisms at the rear of the front will have no oil and will either become dormant or will feed on each other. This effectively regulates the thickness of the biomass layer ensuring that the vitamins, phosphates and the electron acceptor penetrate to the forward part of the layer, allowing fresh oil to be acted upon. Thus, the front advances through the oil towards the outlet and dislodged oil is constantly being flushed out by the injection water.
By a process of natural selection, only the most successful micro-organisms thrive and these will be the ones most effective in using the oil. They will therefore be the most efficient at dislodging the oil, probably by the production of surfactants. However, due to the flushing action of the injection water, the displaced oil is removed and so only a very small proportion of the oil will actually be consumed by the biomass.
One theory as to how the oil is dissociated is that the oil is split into small droplets by the surfactants and these are washed out. However, the present applicants believe that the oil is initially disposed in long strands or ribbons in the rock pore structure and that the surfactants begin to affect only portions of these strands. In this way, the overall viscous forces attaching a strand will be reduced and the injection water pressure eventually dislodges the entire strand, rather than its being broken up by the surfactants.
The micro-organisms may be any convenient anaerobic and/or facultative anaerobic bacteria. Suitable bacteria may be SRB, NRB, IRB and acetogenic bacteria. The bacteria used may be pre-selected and cultivated to thrive in the injection water under the prevailing conditions.
Examples of suitable nitrates and phosphates include NaNO3, KNO3, NH4NO3, Ca(NO3)2, Na2HPO4, and K2HPO4. If sulphates are relied upon, these would be naturally present in sea water.
It will be appreciated that by using a system according to the invention, the advancing biomass layer may remove efficiently all the oil it encounters. Either the oil will be dislodged and flushed away or it will be consumed and converted though some may remain. The liberated oil can be separated from the water, minerals and organic material by conventional methods though it is desirable to minimise any pre-separation exposure to air in order to avoid further microbial action on the oil.